Method for operating an alternating pressure apparatus for producing oxygen from the air

ABSTRACT

An alternating pressure apparatus for obtaining oxygen from the air has two adsorbers to which an air blower for adsorption air and a vacuum pump stand for desorption are connected. The vacuum pump stand has two vacuum pumps in tandem. In a partial load operation characterized by low O 2  removal; and during the lower pressure equalization of the adsorbers, the vacuum pump stand is connected to the air blower in that in each case the vacuum valve, connecting one adsorber to the vacuum pump stand, and the air valve, connecting the same adsorber to the air blower, are opened at the same time.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German application No. 196 51457.6, filed in Germany on Dec. 11, 1996 and German application No. 19725 678.3, filed in Germany on Jun. 18, 1997, the disclosures of whichare expressly incorporated by reference herein.

This invention relates to an apparatus for obtaining oxygen from theair. More particularly, this invention relates to an alternatingpressure apparatus for obtaining oxygen from the air, which has at leastone adsorber containing synthetic zeolite to which an air blower for theadsorption air and one vacuum pump stand for the desorption of theadsorber are connected for alternate operation, the vacuum pump standhaving two vacuum pumps in tandem. The invention furthermore relates toa method for the operation of such an alternating pressure apparatus.

An alternating pressure apparatus of the type described above, whichserves, however, for the production of nitrogen from the air, isdescribed for example in DE-A-31 46 189. In the vacuum pump stand of theknown alternating pressure apparatus, between the two vacuum pumps thereis disposed an intermediate cooler configured as a heat exchanger, whichoperates dry, in contrast to the cooling by water injection that isgenerally used, and which cools the air compressed by the first vacuumpump to the extent that no overheating occurs in the second vacuum pump.The second vacuum pump is provided with preliminary inlet cooling. Thealternating pressure apparatus according to that disclosure operateswith adsorption pressures between 1 bar and 2.5 bar and desorptionpressures of 85 mbar.

In alternating pressure apparatus for obtaining oxygen, operation atpartial load often occurs, because for a period of time only a reducedamount of oxygen is needed. To save energy in this case it is commonduring the dead periods to run the vacuum pump stand on bypass. For thispurpose a comparatively complicated bypass plumbing, bypass valves andcontrol means for the purpose are necessary, which involves considerableexpense.

The present invention solves the problem of designing an alternatingpressure apparatus of the kind referred to above such that it will beable to operate in an energy efficient manner when operating at partialload with a minimum cost of construction. Furthermore, a method foroperating such an alternating pressure apparatus is provided.

The problem first mentioned is solved according to the invention in thatin each adsorber only one line common to pressure and vacuum runs from adistribution line, that on both sides of the point at which the linesare connected to the distribution line an air valve and a vacuum valveare inserted into the distribution line, that the vacuum pump stand hasa connection to both outer ends of the distribution line, and the centerof the distribution line between the air valves has a connection througha line to the air blower.

Thus the possibility exists, with very little use of piping and valves,for connecting the air blower to the input of the vacuum pump stand.This reduces the pressure difference in the vacuum pump stand to about<200 mbar, so that without the cost of a complicated bypass for thevacuum pump stand it is possible to run at partial load withcorrespondingly reduced energy demand.

In alternating pressure apparatus for the production of oxygen, however,the cooling of the air by water injection has become widespread, becauseit was assumed that the vacuum pump stand with the operation of the twovacuum pumps can be virtually isothermic and therefore especially energyefficient.

The injection of water into the oxygen-depleted air aspirated by thevacuum pump stand assumes that extremely clean processed water free ofminerals is used so as to avoid malfunction of the vacuum pumps. Often,however, no water or no suitable water is available for injection, sothat apparatus using water injection are out of the question andsomewhat less energy-efficient apparatus are given preference, likethose described in the above-mentioned DE-A-31 46 189. But since suchapparatus require water for cooling the air, their employment involvesdifficulty in many places. Dispensing with cooling water or injectionwater has not been possible heretofore, however, because the vacuumpumps, configured as rotary piston pumps, tolerate a maximum temperatureof about 125° C. to 130° C., and in the known apparatus the aspiratedair, unless cooled, would be heated by the compression in the vacuumpump stand to about 200° C. after the second stage.

The alternating pressure apparatus of the instant invention is able tooperate in an energy efficient manner without water injection or watercooling and at temperatures safe for rotary piston pumps if according toan embodiment of the invention, the second vacuum pump in the directionof flow is a rotary piston pump with preliminary inlet cooling, havingcooling air inlet ports for the defined inflow of outside air ofatmospheric pressure, that the inlet connection of the second vacuumpump, without the use of gas cooling by water injection or by heatexchanger has a connection to the outlet of the first vacuum pump, andthat the vacuum pump stand is provided with control systems forproducing a desorption pressure between 300 mbar and 500 mbar.

Since according to the invention higher desorption pressures are usedthan in the state of the art, relatively low compression work occurs inthe second vacuum pump operating against the atmosphere. Due to theadmixture of air from the atmosphere, the heat thus produced causeslittle or no temperature rise, so that the vacuum pump stand can operateentirely without water injection or water cooling. Surprisingly,measurements on a constructed alternating pressure apparatus accordingto the invention showed that, at the same oxygen yielding power as inknown alternating pressure apparatus, the energy required is even lower.

An especially low energy requirement is achieved if, according to anadvantageous embodiment of the invention, the cooling air inlet ports ofthe second vacuum pump are of such dimensions and positions that theratio of the total mass flow of the aspirated mass flow of the firstvacuum pump and of the cooling air mass flow to the cooling air massflow at the start of the desorption cycle drops rapidly from a highlevel to 33 and during the desorption cycle slowly drops to 1.5 by theend.

It is important to the cooling principle according to the inventionthat, in the rotary piston pump operating as a vacuum pump, the rotarypistons block the connection to the inlet connection before the workingpump chamber receives a connection to the atmosphere and therefore theair can flow in and perform compression work. This can be achievedeconomically because, according to another embodiment of the invention,the cooling air entry ports in the second quadrant begin not before 36°and end before 90°.

The fundamental frequency of the pulsations can be doubled, so thatsimple pulsation dampers can be used if the cooling air inlet ports areconnected by a common line to the atmosphere.

Since the air blower for the adsorption air and the vacuum pump standproduce very bothersome noise in operation, these parts of apparatus areusually disposed in a noise-suppressing box container. Therefore a greatdeal of expense is involved in cooling with a positively driven airstream. This expense can be kept comparatively low if, according toanother embodiment of the invention for cooling the apparatus, a blowerof an air cooler necessary for the adsorption air is provided, and if acooling air stream is guided along the vacuum pump stand and theadsorption air blower to the blower.

An excessively low temperature of the air entering the adsorber can beprevented simply by providing the blower with a controlled-speed fanmotor. In this manner the amount of cooling air can be reduced at lowexternal temperatures by reducing the speed of the blower.

The alternating pressure apparatus is very economically constructed if aradial blower with a controlled throttle in its intake line is providedas the first vacuum pump.

It is also desirable if only one line common to the positive pressureand the negative pressure runs from a distribution line or manifold intoeach adsorber, if an air valve and a vacuum valve are inserted into thedistribution line on each side of the point where the lines areconnected to the distribution line, and if the air blower or the vacuumpump stand is connected to the two outer ends of the distribution lineand a line from the vacuum pump stand leads into the middle of thedistribution line between the air valves. In this manner it is madepossible with very little expenditure on piping and valves to connectthe air blower to the inlet of the vacuum pump stand when operating atpartial load. Thus the pressure difference of the vacuum pump stand isreduced to about 200 mbar, so that without the expense of a complexbypass for the vacuum pump stand it is possible to operate at partialload with a correspondingly lower energy demand.

Back flow of oxygen from the buffer tank through the adsorbers and thenthrough the vacuum pump stand to the atmosphere when running at idle,when the air blower is connected to the inlet of the vacuum pump stand,can very easily be prevented by providing a shut-off in the productdelivery line.

The shut-off is of very simple construction and does not need to beoperated in harmony with the adsorber outlet valves if it is a checkvalve opening toward the storage tank.

When running at partial load it is possible to operate entirely withoutthe first vacuum pump in the form of a radial blower, and thus save muchenergy, if, according to another embodiment of the invention, the firstvacuum pump in the form of a radial blower has a bypass line connectingits suction line directly to the second vacuum pump in the form of arotary piston pump, and containing a check valve port toward the secondvacuum pump.

For further improvement of the energy balance it is helpful if, toevacuate the first vacuum pump, configured as a radial blower, anauxiliary pump is provided. Thus the radial blower can run partiallyloaded without having to perform work.

The vacuum pump stand is able, independently of the air blower, toaspirate air from the atmosphere if an air inlet line having a checkvalve closing toward the atmosphere is inserted into the line connectingthe air blower to the adsorbers. Such a configuration, however, isnecessary only in exceptional cases.

The second problem referred to, namely the creation of a method foroperating an alternating pressure apparatus with the above-describedfeatures, is solved by the invention in that at partial load operationcharacterized by low oxygen uptake, the adsorption pressure is increasedabove the adsorption pressure at full-load operation, and when a set topadsorption pressure is reached, the air blower for the adsorption air isswitched ahead of the two-stage vacuum pump stand. With this procedureit becomes possible with little investment to operate at partial loadvery economically.

Partial load operation can be terminated with especially little effortif the pressure at the storage tank or buffer tank is monitored, andwhen it falls below a set level, the air valves and vacuum valves areswitched back to cyclical adsorption and desorption. The pressure dropthat is measured indicates that oxygen uptake is again increasing.

The use of the alternating pressure apparatus of the present inventionis desirable in regions without sufficient high purity water when arotary piston pump with precooled inlet is used as the second pump inthe direction of flow, and if a controlled flow of outside air atatmospheric pressure is added, when the gas leaving the first vacuumpump is fed to the inlet of the second vacuum pump without theinterposition of gas cooling by water injection or the use of a heatexchanger, and when the vacuum pump stand is operated at a desorptionpressure between 300 mbar and 500 mbar.

With a method of operation of this kind it is possible to produce oxygenwithout cooling water or water injection and with an energy consumptiondecidedly lower than in the known apparatus.

It is especially good from the energy viewpoint if, according to apreferred embodiment of the invention, the cooling air inlet ports ofthe second vacuum pump are of such dimensions and positions that theratio of the total mass flow of the aspirated mass flow of the firstvacuum pump plus the cooling air mass flow to the cooling air mass flowat the start of the desorption cycle falls rapidly from a high level atthe start of the desorption cycle to 33, and within the desorption cycleslowly sinks to 1.5 by the end.

It has been possible to learn by experiment that energy consumption isespecially favorable if, according to another embodiment of the method,an adsorption pressure between 0.7 bar and 1.8 bar, preferably between0.9 bar at the beginning and 1.5 bar at the end of the adsorption isselected along with a desorption pressure between 350 mbar and 400 mbar,and if the starting pressure for the adsorption and desorption after thepurging phase and the pressure build-up phase amounts to a minimum of700 mbar and a maximum of 950 mbar.

After a changeover to building up pressure, in order to be able toachieve a build-up to adsorption pressure in an adsorber, according toanother embodiment of the invention the pressure is throttled by meansof a throttle in the product delivery line behind the buffer tank.

It is also beneficial to the operation of the alternating pressureapparatus if the throttle is followed by a buffer tank and the capacityof the latter is such that the pressure in the buffer tank variesbetween 1.2 bar and 1.5 bar within an adsorption cycle.

The adsorption air blower and the vacuum pump stand are cooled by apositively guided air stream, without the danger of an excessively lowinput temperature in the adsorber at low outside temperatures, if ablower of an air cooler required for the adsorption air is used forcooling the parts of the apparatus, if a cooling air stream is carriedalong the vacuum pump stand and the adsorption air pump stand to theblower, and if the blower is controlled by a frequency-controlled fanmotor such that the temperature of the air entering the adsorber doesnot fall below a minimum level. Experience has shown that the oxygenconcentration is considerably reduced if the adsorption temperaturesfall below 20° C. By the procedure of the invention, decreasing outsidetemperatures can be compensated by reducing the volume of the coolingair stream.

If the outside temperature is so low that reducing the speed of thecooling air blower is insufficient to maintain adsorption temperaturesabove 20° C., it is possible according to an embodiment of the inventionto use the hot air put out by the vacuum pump stand to raise thetemperature of the air from the air blower. Thus, a throttle valve isinstalled ahead of the air blower so that some cold air is aspiratedfrom the container housing the vacuum pump stand, and another air streampreheated by the vacuum exhaust through a heat exchanger is aspirateddirectly at the exhaust air exit. By this the temperature level afterthe air blower is raised accordingly, so that the 20° C. adsorptiontemperature can be maintained by controlling the speed of the motordriving the cooling air blower. This preheating is possible because thesecond vacuum pump according to the invention operates without waterinjection and thus operates at higher temperatures than in the state ofthe art.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawingswherein:

FIG. 1 is a schematic diagram showing the circuit of an alternatingpressure apparatus according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a PVSA (pressure vacuum swing adsorption apparatus)according to the invention, which operates with synthetic zeolite in twoadsorbers 1, 2, connected in parallel. Air is fed alternately to theseadsorbers by means of an air blower 3 in the form of a rotary pistonpump in a pump stand 17. The blower 3 furthermore serves to produce andmaintain the necessary adsorption pressure. For this purpose a line 4connects the air blower 3 to a distribution line or manifold 5 intowhich two vacuum valves 6, 7 are inserted on the outside and two airvalves 8 and 9 on the inside. The line 4 opens into the distributionline 5 between the air valves 8 and 9, and lines 10 and 11 run frompoints between vacuum valves 6 and 7 and air valves 8 and 9,respectively, to the adsorbers 1 and 2.

For the alternating evacuation of the adsorbers 1 and 2 and thus foraspirating the nitrogen-enriched fraction from the adsorbers 1 and 2, avacuum pump stand 12 is used, which has two vacuum pumps 13 and 14 intandem. The vacuum pump 14, which forms the second pump stage and isdisposed on the atmosphere side, is a rotary piston pump with apre-cooled inlet, which will be described later in conjunction with FIG.2. Into this second vacuum pump 14 air flows from the atmosphere througha line 15 to cool it.

The vacuum pump stand 12 is connected by a suction line 16 to the twoouter ends of the distribution line 5. The vacuum valves 6 and 7 and airvalves 8 and 9 make it possible to feed compressed air from the airblower 3 to the suction side of the first vacuum pump 13 of the pumpstand 12 during part-load operation.

The oxygen is delivered from a storage tank 18 which is preceded by ablower 19 or compressor, so that the oxygen comes out under pressure.The blower 19 or compressor is preceded by a buffer tank 20 into whichthe oxygen is fed with a purity of up to 95% through a shut-off valve21. The shut-off valve is a check valve that opens from the adsorbers 1and 2 to the storage tank 18.

The outlets of the two adsorbers 1 and 2 are connected together by aconnecting line 22 into which there is inserted a shut-off valve 23 anda throttle 24. By means of this connecting line 22, the shut-off valve23 and the throttle 24, the two adsorbers 1 and 2 are connected to oneanother for purging by the adsorber 2 or 1 that is still in adsorption,purged with the air blower 3 and vacuum pump stand 12 running, and thusa first build-up of pressure in the regenerated adsorber to about 500mbar is achieved. After this purge the product delivery valves 25 and 26in lines 27, 28, which run to a product removal line 29 with the checkvalve 21, are opened. By the simultaneous opening of the air valve 8 or9 associated with the particular adsorber 1 or 2, the latter is thenconnected to the air blower 3 as well as to the pump stand 12, so thatanother pressure build-up can take place or a pressure equalization inboth adsorbers 1 and 2, top and bottom.

The lower pressure equalization operates precisely the same as runningat partial load, but this function is very brief and always is repeatedcyclically. For example, if adsorber 1 is regenerating and adsorber 2 isadsorbing, the lower valves are operated as follows: valve 6 open, valve8 closed, valve 9 open and valve 7 closed. During the lower pressureequalization, valve 8 is additionally opened within a certain amount oftime within the pressure equalization period. Then valve 7 is opened andvalve 8 closed, and after the pressure equalization period ends, valve 9is closed. This assures that, during the lower pressure equalization aconnection of the vacuum pump stand 12 to the air blower 3 is maintainedexactly as during partial load operation. The advantage is that, withinthe pressure equalization time the valves are brought to the correctposition for the next adsorption and desorption, so that the periodsduring which no production takes place are especially short.

With the kind of pressure equalization described, the lower pressureequalization is performed simultaneously with the upper pressureequalization. Thus nitrogen is aspirated away downwardly from theparticular adsorber. Due to the pressure equalization, it is not thenitrogen but mostly air that flows into the regenerated adsorber fromthe first adsorber.

After the pressure has built up from 500 mbar to 900 mbar, thecorresponding vacuum valve 7 or 6 of the regenerated adsorber 1 or 2 isclosed, and at the same time the corresponding product delivery valve 25or 26 is closed for the adsorber 1 or 2 that is to be regenerated, sothat the adsorption phase takes place in the regenerated adsorber 1 or 2and the desorption phase in the adsorber 1 or 2 that was previously inadsorption.

In the pump stand 17 with the air blower 3 only one air cooler 32operating with air as coolant has been indicated, with which a blower 30is associated by which a stream of cooling air 31 is first aspirated bythe air cooler 32 at one end of the vacuum pump stand 12, then back toits other end, and then along the pump stand 17 for the adsorption air.The usual components, such as filters, sound mufflers, a safety valveand valves have not been indicated. The air cooler 32 serves to recoolthe air compressed to a maximum of 1.5 bar by the air blower 3 in theform of a rotary piston blower. This pressure is set by the amount ofproduct withdrawn and the throttle 40 behind the buffer tank 20, sothat, in the adsorber 1 or 2 that is adsorbing, the desired maximumpressure is reached after each adsorption phase. The adsorptiontemperature is controlled by controlling the speed of blower 30 by meansof a frequency-controlled fan motor 46.

In the case of vacuum stand 12, the position of common components hasnot been indicated, either. It is important to the invention that thecompressed, hot and oxygen-depleted air at 50° C. to a maximum 110° C.coming from the first vacuum pump 13 is fed directly in each cycle tothe second vacuum pump and aspirated at this temperature from the secondvacuum pump.

In FIG. 1, furthermore, there is shown a bypass line 41 through whichthe second vacuum pump 14 can be connected directly to the adsorbers 1and 2 without going through the first vacuum pump 13. A check valve 42is inserted into this bypass line 41, which opens toward the secondvacuum pump 14. An auxiliary pump 43 can be provided for the evacuationof the first vacuum pump.

In order to connect the inlet side of the vacuum pump stand 12 to theatmosphere even while the air blower is not running, an air inlet line44 with a check valve closing toward the atmosphere leads into line 45.

As FIG. 1 shows, line 15 branches into hoses 38 in order to connect bothof the cooling air ports 33 and 34 to the atmosphere. Thus, pulsationsof twice the fundamental frequency occur in line 15, so that simplepulsation dampers suffice.

The alternating pressure apparatus described operates preferably at atotal cycle time of 2×33 to 2×45 sec. Thus from 7 to no more than 15seconds fall to the flushing and pressure equalization phase. Thisflushing and pressure equalization phase is performed whenever thealternately occurring switchover from adsorption to desorption takesplace in the one adsorber 1 or 2 and from desorption to adsorption inthe other adsorber 1 or 2.

Lastly, it should be noted that all of the pressures specified areabsolute pressures.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method for the operation of an alternatingpressure apparatus for obtaining oxygen from the air,comprising:alternately applying pressure to two adsorbers by an airblower for the adsorption of nitrogen and a vacuum for desorption,wherein a vacuum pump stand with two vacuum pumps connected in tandem isused for producing the vacuum, wherein in a partial load operationhaving low O₂ removal, and during a lower pressure equalization of theadsorbers, said method further comprises: connecting said vacuum pumpstand to the air blower by opening a vacuum valve connecting oneadsorber to the vacuum pump stand and an air valve connecting said oneadsorber to the air blower at the same time.
 2. A method according toclaim 1, further comprising:in the partial load operation, raising theadsorption pressure above the adsorption pressure at full loadoperation, and upon reaching an established upper adsorption pressure,connecting the air blower for the adsorption air ahead of the pumpstand.
 3. A method according to claim 1, wherein the apparatus includesa storage tank and a buffer tank connected to the adsorber and airvalves and vacuum valves for controlling adsorption and desorption, saidmethod further comprising:monitoring the pressure at the storage tank orbuffer tank and, when it falls below a set level, switching the airvalves and vacuum valves back to cyclic adsorption and desorption.
 4. Amethod according to claim 3, wherein the apparatus further comprises athrottle in the product delivery line after the buffer tank to therebyenable pressure build-up to adsorption pressure at a desired time afteran adsorber is switched to pressure build-up.
 5. A method according toclaim 4, wherein the capacity of the buffer tank is such that theadsorption pressure in the buffer tank varies between 1.2 bar and 1.5bar within an adsorption cycle.
 6. A method according to claim 5,wherein a blower of an air cooler necessary for the adsorption air isused for cooling parts of the apparatus, said method furthercomprisingdirecting a cooling air stream along the vacuum pump stand anda pump stand for the adsorption air to the blower, and controlling theblower by means of a frequency-controlled fan motor such that thetemperature of the air entering the adsorber does not drop below abottom value.
 7. A method according to claim 6, further comprising, atlow outside temperatures, using the hot exhaust air of the vacuum pumpstand for raising the temperature of the air from the air blower.
 8. Amethod according to claim 1, wherein the lower pressure equalizationtakes place at the same time as an upper pressure equalization.
 9. Amethod according to claim 1, further comprisingselecting an adsorptionpressure between 0.7 bar and 1.8 bar, and a desorption pressure between350 mbar and 400 mbar, and wherein the starting pressure for theadsorption and desorption after flushing phase and pressure build-upphase amounts to a minimum of 700 mbar and a maximum of 950 mbar.